This invention relates to refrigeration systems and more particularly to a seal-less expander for use in split Stirling cycle refrigerators.
The need for cooling infrared detectors to cryogenic temperatures is often met by miniature refrigerators operating on the split Stirling cycle principle. As is well known, these refrigerators use a motor driven compressor to provide a pressurized refrigeration gas with a nearly sinusoidal pressure variation, an expander to cool the gas, and a gas transfer line to feed the pressurized gas from the compressor to the expander.
The expander is typically a metal cylinder with two isolated cavities formed therein. One cavity, called a spring volume, is filled with refrigeration gas held at a nearly constant pressure. The other cavity, called the working volume, is also filled with refrigeration gas and receives the gas fed from the compressor via the transfer line. The pressure of the gas in the working volume is thereby made to oscillate above and below the spring volume pressure. Another metal cylinder called a displacer is disposed within the expander so that one end of the displacer extends into the spring volume and the other end extends into the working volume.
The displacer is free to reciprocate in response to the varying pressure of the gas in the working volume. The displacer reciprocates because when the spring volume pressure is less than the working volume pressure, a force on the displacer is created causing it to move so that more of the displacer extends into the spring volume. When the spring volume pressure exceeds the working volume pressure, the displacer is again forced to move in the opposite direction, out of the spring volume and into the working volume.
The displacer also contains a regenerative heat exchanger (regenerator). Openings formed in the displacer allow the gas in the working volume to access the regenerator. As the displacer reciprocates, working volume gas is forced to flow through the regenerator first in one direction and then the other. This in turn causes the gas to be alternately cooled and warmed, so that gas in one end of the working volume (a cold space) becomes colder than ambient, and gas in the other end (a warm space) becomes warmer than ambient. An infrared detector or other device to be cooled is thus mounted adjacent the cold space in the working volume.
It is known that efficiency of the expander can be increased by retarding the movement of the displacer until the pressure of the gas in the working volume is near maximum. This allows almost all of the gas in the working volume to be compressed and thus warmed before being forced to flow through the regenerator. The gas is then as cold as possible when entering the cold end of the working volume. Likewise, retarding the opposite motion of the displacer until the pressure in the working volume is near minimum causes as much gas as possible to be expanded and hence cooled before being forced out of the cold end and back through the regenerator.
Retarding the motion of the displacer is primarily the function of a displacer friction seal disposed so as to contact the outer diameter of the displacer. A major problem has been that these friction seals change their braking action unpredictably as they wear, thereby adversely affecting cooling efficiency. U.S. Pat. No. 4,074,908 to Spencer discloses a polymeric seal exhibiting long wear life. Others have replaced the friction seal with a clearance type seal and provided other means for delaying or trimming the motion of displacer 14. For example, U.S. Pat. No. 4,514,987 to Pundak, et al. discloses the use of an electromagnetic field to produce mechanical drag on the displacer. U.S. Pat. No. 4,475,346 to Young, et al. discloses an electrically powered linear drive motor for trimming the movement of the displacer. In addition to increasing the mechanical complexity of the expander, these auxiliary motion retarding mechanisms increase the necessary number of external connections to the expander. This in turn complicates the mounting of the expander and the infrared detector on the gimbals necessary for steering.